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    A selfspanning crossbow in the Loeffelholz MS

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    mac
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    A selfspanning crossbow in the Loeffelholz MS

    Post by mac on Mon Mar 16, 2015 9:05 pm

    First topic message reminder :

    Here are a couple of details from pages in the Loeffelholz manuscript of 1505. http://jbc.bj.uj.edu.pl/dlibra/docmetadata?id=258834&from=publication

    They show a pretty cool way to make a selfspanning bow. Sorry about how I cropped one of the nocks.  The images from the Biblioteka Jagiellonska are DjVu format and I had to use screen captures to get them into Jpeg. Unfortunately,  the zoom was in set increments.   Perhaps a Geekier fellow would have a better way....


    In any case, here it is.  As military tretises go, Loeffelholz is very reliable.  So, who knows... this might even work.





    Mac

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by Rizzar on Thu Mar 19, 2015 1:08 pm

    I used to have no problem using CAD myself.
    Good for mechanism proportions.
    Especially when it comes to multiaxles or even spanning mechanisms ( I created my whole cranequin on inventor before doing a single step)
    I agree real time forces simulation would be overkill.

    The mechanism is not that difficult I think, it´s just about hidden optional details.

    Coincidentally the creator of that piece posted today on the german community I used to visit. ^^

    So I think details are not very hard to get the next days Very Happy
    Going to keep you updated when there is sth new to report.
    He mentioned the 100kg max due to the lever dimensions.

    Rizz

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by edstuff on Thu Mar 19, 2015 1:13 pm

    I've gotta take some time to really look it over to at least get some basic dimensions to start from but it might be a fun undertaking

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by Rizzar on Thu Mar 19, 2015 1:16 pm

    Remember dimensions strongly depend on the used prod, so this would not be an universal template.

    mac
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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by mac on Thu Mar 19, 2015 1:21 pm

    I started some little thumbnail sketches in my notebook a few days ago.  If I were intent on building it, my next step would be full size drawings and simulations using Cardboard Aided Design.  My usual "CAD package" includes brass paper fasteners, a small leather punch to make holes, some chipboard and a pair of scissors.

    I look forward to learning what Herr Bichler has to say about the limitations of the design.  

    Mac

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by mac on Thu Mar 19, 2015 1:33 pm

    If you started with a brace height, and a draw length, you could work out the geometry of the spanning mechanism.   You would need to know the draw weight and dimensions of the center of the bow in order to make the parts strong enough to bear up. 

    One of the things that worries me about the design as it appears in the MS is that the hole for the binding cords must end up lower than its ideal position in order to make room for the "carriage".   The other is the lack of solid wood above the bow.  We know that this is a place where normal tillers sometimes fail.  I think I would be tempted to put some iron reinforcements in the "bow socket" to make up for the support it will not get from the carriage.

    Mac

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by edstuff on Thu Mar 19, 2015 1:46 pm

    mac wrote:I started some little thumbnail sketches in my notebook a few days ago.  If I were intent on building it, my next step would be full size drawings and simulations using Cardboard Aided Design.  My usual "CAD package" includes brass paper fasteners, a small leather punch to make holes, some chipboard and a pair of scissors.

    I look forward to learning what Herr Bichler has to say about the limitations of the design.  

    Mac

    I literally laughed out loud at "cardboard aided design"
    If you wouldn't mind sharing your pencil sketches I'd be happy to CAD them!

    mac
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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by mac on Fri Mar 20, 2015 8:13 am

    Here's what I've sketched.  I guarantee nothing.  Most of the sketches are "mid sagittal sections" with some ghosts of things that are out of that plane.

    In general, I think that the overall height of the carriage needs to be reduced to something just barely tall enough to contain the nut and sear bar.  This will allow more wood to be added to the tiller, and that will let us bring the bow binding hole up a bit.   Removing material from the carriage should not be a problem because the iron binding is carying the load. There just has to be enough wood to bed the bearing blocks. 

    The important thing about the mechanism is this.  The lengthwise travel of the spanning bar pivot determines the distance that the carriage will travel.  That has to be equal to the intended "power stroke" of the bow. 

    The iron bindings of the carriage are primarily in tension from the rear pivot to the forward bearing block of the nut,so they do not have to be very thick. By contrast, the spanning bar is always in compression, so it must be thick enough not to buckle.  Pursuant to that, the articulation of each end of the bar must not permit any twisting or deflection. To that end, the fit with the iron straps of the carriage must be accurate, and that goes for the mortise in the wooden lever as well.  The pivots must be large enough in diameter to bear the shear load imposed by the spanning bar.  Likewise, there must be enough wood around the forward pivot to support the load. The artist has shown the forward pivot within an area of horn inlay, and that's probably a good idea. 

    The MS drawing seems to show the hinge held on by clinched nails.  That's one of those details that lends a lot of verisimilitude to the drawing. The artist has not shown them on the lever side of the hinge, but they have to be there as well.   The pivot of the tickler will end up near those nails, so that is a thing to plan around.

    There must be a channel in the bottom of the carriage to allow the "post" of the ticker to enter while the carriage is back. 

    I have not considered where the tickler return spring will be.  Nor have I considered how the tickler's travel is limited.

    I have not considered the nature of the catch at the end of the tiller.  There must be a sliding member and it must have a spring. 

    This is, of course, just for starters.  There is a lot to think about here.



    Mac

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by mac on Fri Mar 20, 2015 10:03 am

    A couple more thoughts....

    When the spanning lever (the lower part of the tiller) is entirely closed, the force of the bow will be born as compression between the wooden parts at the joint.  However, for the entire spanning stroke, all of the force of the bow will be experienced by the hinge. The force on the clinch nails will mostly be shear, but there will be some tension on them as well.  Inletting the hinge leaves into the wood will help some if done accurately. 


    At the forward end of its travel, the nut should end up in a position to receive the bowstring.  There are two ways this can work.  The first is by manually "popping" the sting over the nut. The other possibility is that the nut is left with the fingers down and the string rotates the nut into position as the nut passes under it.  The shooter would then press the button behind the nut to make sure the sear bar was seated in the notch of the nut. 

    Because the two pivot points of the spanning bar never end up in a line parallel to the track of the carriage, there is always a force trying to open the two parts of the tiller.  Thus, it is very important that the catch on the butt of the tiller is secure.  If it fails, the spanning lever will swing down forcibly, releasing the energy stored in the bow.  Injury to the shooter, and the crossbow, could be expected under those circumstances.

    Mac

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by Rizzar on Fri Mar 20, 2015 10:37 am

    Short update of the things I learned from Andreas Bichler so far:

    Force
    Problems concerning draw weight depend on two things
    1- Lever strength and length, he simply felt 100kg to be his maximum. He tried 120 but it was not good.
    2- He always tries to get as close to the original as possible (reconstruction), and the pictures show a really thin tiller. He states that on a higher draw weight problems could likely occur.

    Mechanism
    He basically confirmed the simple mechanism mac already posted (I had it in mind, too when I asked for it) with the exception of the "nut release button" to be working in push and pull.
    He uses springs to support the button and the trigger bar.

    Safety
    None special except spring preventing trigger bar to be accidentally triggered while loading.

    @MAC: To your second point of the last post. He describes the second situation to prepare the nut with the fingers down, while rotating it in by the string and securing it eventually by the sear bar pushed up by a small spring.


    The butt catch mechanism should be good secured by a metal sheet with a spring, but for the case to unload without shooting not extremely tight.

    For the ones that want to follow the german forums in case of further development/possible pictures Link

    Greetings
    RiZZ

    mac
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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by mac on Fri Mar 20, 2015 11:27 am

    Thank you for that link, Rizz!

    I am taking the liberty of hotlinking to an image from that site.



    I wish we had some pics of Herr Bichler's bow that showed it off better. 

    Mac

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by edstuff on Fri Mar 20, 2015 6:15 pm

    Oh man that is so awesome

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by chaz on Sat Mar 21, 2015 2:25 am

    This is all very interesting ................ edstuff if you can post your "cad" info on this  would be nice..........however, Mac your abilities to CAD " cardboard aided design" is very understandable and from my view certainly nothing to be scoffed at. This is good stuff.

    Chaz

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by edstuff on Sat Mar 21, 2015 11:33 am

    Oh I'll be posting the drafts out so they can be critiqued and changed.  Hopefully when it's all done I'll be able to post a print anyone can use.  Even someone like me Smile

    Edit: we are busy at work though so ya gotta give me time.

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by mac on Tue Mar 24, 2015 11:46 am

    Thinking bout this further, I see that my earlier thoughts about what happens as the spanning lever (lower part of tiller)  is brought to its closed position were in error.   The important thing is that the forward pivot of the spanning bar ends up above a line drawn from the hinge to the rear pivot.  That means that the mechanism is not always trying to open back up, as I predicted earlier.  As the forward pivot passes that line, the leverage becomes theoretically infinite, and then begins reversing.  Thus, there should be a slight force holding the two parts of the tiller closed.  This is a better situation than I thought.

    Mac

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by edstuff on Wed Mar 25, 2015 8:04 am


    OK fellas here's the first cad draft. It was all I could do in between work projects. Critique it, mark it up, make it better.  Then I'll re-draft it.

    mac
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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by mac on Thu Mar 26, 2015 8:10 am

    I'm spending a lot of time on my armor project these days, but I still let myself think about this bow now and then as a way to relax.

    Here's a bit more doodling on the same notebook page.  I had a couple of thoughts.  The one was about how there is a force trying to push the carriage up through the "roof" of the tiller.  This force is at its maximum at the beginning of the spanning stroke, and becomes quite small at the end.

    The other thing is about reducing the material of the forward end thick part of the carriage where the nut is.  Since the iron straps are going to bear the tensional load, I thought that the forward nut bearing might be pinned in place, and no wood would be necessary beyond that.  This would allow for more wood to be added to the front end of the tiller to support the bow.  The pins would be loaded in shear, so they would have to be thick enough to do the job.  I suspect that 5/52" is adequate and 3/16" would be more than enough. The ends of the pins would have to be peened up into countersinks in the iron straps to make the sides of th carriage smooth.  This is not the strongest way to rivet things, but the tensional loads on the pins will be quite small. The stippled sections are to be understood as bone or antler, although I have not used that convention for the nut.

    This sketch includes more detail of the lock, including a spring which is held in place by a shoulder on the threaded end of the reset button.  I am not sure how I feel about the spring.  Apparently Andreas Bichler has one in his reconstruction, but it sort of makes the reset button redundant.  As I have sketched it, it would be easy enough to remove it if it turned out to be unnecessary.

    In the middle of the page, I am trying to think about the catch mechanism, and coming up sort of blank. As I have sketched it, it needs to be set in a very deep narrow mortice.  Well, that's a lot of trouble for a thing like this, so I am clearly missing the boat.   I think I need to see more catches on boxes and such to direct my thoughts. 

    The little sketch on the left posits the idea that the Loeffelholz artist might be mistaken about the relative proportions of the front end of the tiller, and that the binding hole might be at a more or less normal height.  I'm not at all sure I hold with this, but I think it could be made to work even if that's not how the original was.

    Mac

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by edstuff on Thu Mar 26, 2015 2:41 pm

    oh boy you are giving me a lot of work to do ha ha.  But when complete I hope to have multiple views on this. Multiple pages if need be too.  One thing I was really good at when I drafted all day was making complicated prints into something even a person with no print reading skills could understand.  Feel free to chicken scratch all over what I drafted so far if you feel the dimensions are off.  
    Even in the slightest.

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by mac on Thu Mar 26, 2015 3:00 pm

    Ed,

    Everyone likes to work differently.  I would not even consider measurements until the whole thing seemed to have the right proportions and the mechanism looked like it was working.

    You might try scaling things to match Herr Bichler's repro and then seeing how that seems to compare to the Loreffelholz drawings.  In any case, I would recommend beginning with the whole thing and letting that drive the sizes of the internal parts, rather than the other way around.

    Mac

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by edstuff on Mon Jun 01, 2015 4:47 pm

    Not to bring up a hopefully not dead topic but have any of you chaps seen this?
    Might be helpful
    https://www.youtube.com/watch?v=tW4lYh8Yg38

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by kenh on Mon Jun 01, 2015 7:57 pm

    I love that Lachet bow.  Now if I could just find a metal worker to make the action!

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by edstuff on Mon Jun 01, 2015 8:09 pm

    What kind of work is needed to be done to make the action on something like this?
    I USED to be a metal worker.  But it's been years since I did any work, ever since the company was sold.  I used to make steel art on my lunch breaks.  I really miss those days.

    Edit: this is an example of a sculpture I made

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    Re: A selfspanning crossbow in the Loeffelholz MS

    Post by edstuff on Tue Jun 09, 2015 10:15 am

    Got a lil bit more drafted. I'll keep posting if I get more done.

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